My adapters are just 14 or 16 gauge plates with the inside shape matching the inside of the Magnum exhaust gasket, and the outside matching the 340 manifold. Then I sandwiched them between a Magnum gasket on the head side and a 340 gasket on the other side.
In my case, they have been a little problematic, too. I expect that at some point I will just spring for a set of Doug's headers and get rid of them. I had to re-gasket them once and the second time around they didn't seal well. Could have been something I did wrong, or could be they don't want to seal more than once.
I have pictures of them installed, but I don't think I have any after I got them cut on a plasma table here at work.
View attachment 1715353158 View attachment 1715353159
Here is a screenshot of the AutoCAD file I used to get them cut. Started with a scan of both gaskets and traced what I needed and merged them together.
View attachment 1715353160